drm/doc: Fix title underline length

The underline length for the new Analog TV properties section doesn't
match the title length, resulting in a warning.

Fixes: 7d63cd85 ("drm/connector: Add TV standard property")
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20221128081938.742410-1-maxime@cerno.tech


Signed-off-by: Maxime Ripard <maxime@cerno.tech>